Concentration of formaldehyde by distillation and fractional condensation



April 13, 1954 A F, MaoLEAN CONCENTRATION OF FORMALDEHYDE BYDISTILLATION AND FRACTIONAL CONDENSATION Filed NOV. 4, 1949 m. w m w.

ALEXANDER F. MACLEAN ATTORNEYS.

Patented Apr. 13, 1954 PATENT orties DSTIIJLTIQN nENsarIoNj niexanderF.Mae-Lean, Rotstown, rex., assigner to. Celauese (loi-.poration.ol?`America, New York, N. Yfya'. corporation of Delaware ApplicationNdvember4, i949, Serial'No. 125,600

GLIGlaims. lf

This inventionfrelates toy theA 'concentration "of aqueous solutions offormaldehyde; and relates more particularly to the'conceritration'fofaqueous `fiormaldehyde solutions-by partial condensa-- tion reactionsand to' the production of paraformaldehyde lfromt-he concentratedformaldefhydes'olutinsthus obtained.

object-of this inventionis the'provislon of an improvedprocess for theconcentration of aqueoussolutions of formaldehyde.

Another object oftliis invention is the provi sion of a'coritinuou'sprocess vfor the concentration of aqueous solutionsfd'f'formaldehydeWhere- 4by concentrated'formaldehyde capable of yield- `ingparaformaldehyde on cooling may be obtained.

A further objectof 'this "invention is the provisionof acondns'roflimproveddesg'n in which the total condensation of concentratedaqueous 'solutions of formaldehyde may l'beeliected Without plugging andlikeffaults "due to the depositionf of paraformaldehyde therein.

Other objects o'fthsinvention will appear from the followingdetailedldescription andthe accompanying drawing.`

In the drawing,

Fig. 41 is a diagrammatic showing o'fthefmanner in which the novelprocess of my invention may be carriedout, and

Fig. 2 is a detail view of a condenser. ftraylor element, a plurality ofwhich, when suitably assembled, comprise the novel'condenser, or'bubblecap column, of my invention.

Like reference numerals indicate like parts throughout both views of`the drawing.

Paraforrnaldehydeisa solid or semi-solidmixture of formaldehyde polymersassociated with varying amounts of water. When paraformaldehyde isdissolved. in water a depolymerization takes place and` aqueousformaldehyde solutions of any desired concentrationl may be readilyobtained. Paraformaldehyde may be produced by evaporating aqueousformaldehyde solutions under vacuum. Asthe-water is removed, the

formaldehyde undergoes a polymerization and,`

upon cooling,paraformaldehyde separates. The average degreeofpolymerization of. paraformaldeliyde `varies with the conditionsemployed for the .production-of the polymer. The production ofparafor-maldehyde of an excessively high degree of vpolymerization isundesirable since highly polymeri'zed paraformaldehyde is not easily andquickly dissolved in water; Para- 'formaldehyde of too low ua degree'rpolymerizaand difficult to vhandle and; furthermore, 'is of `a tainedl`treated to yield paraforinaldehyde of a substantially uniform andcontrolled degree of polymerization if the said aqueous formaldehydesolutions are vconcentrated in the vapor phase by means of a pluralityof partial condensations, effected in series,vand the concentratedformaldehyde vapors obtained totally condensed and Vthe condensatecooled so that paraformaldehyde is produced therefrom.

Thus, in accordance with my Ynovel process, relatively dilute aqueoussolutions of formaldehyde from any suitable source, such as theoxidationof methanol or the oxidation of hydrocarbons, containing fromabout l to about 30% by weight of formaldehyde are distilled under apressure oi" about 30 to 45 pounds per square inch gage with the liquidphase being at a temperature of l vto 140 C., Vthe distilled vapors arepassed through a pressure reducer Where the pressure is broughtdowntoabout 5 to 15 pounds per" square inch absolute, and the vaporsthen subjected toa plurality'o partial condensations. The initialpartial condensation is effected so as to condense about 3G to 50% ofthe vapors. The vapors may then be conveniently separated from thecondensate by meanscof a suitable cyclone separator and the condensate,which is at a tern;- perature of '72 to 100 C. and which may contain 5to 15% by Weight of formaldehyde, returned `to the system. Theuncondensed vapors are subjected to a second partial condensationduringwhich about to 30% of the vapors remaining are partially condensed.After this condensate, which is at a temperature of 'Z0 to 95 C., lisseparated from theuncondensed vapors in a second cyclone separator, thevapors remaining are subjected to a third partial condensation. Afterseparation of the condensate which is at a temperatui'e of 50 to 80 C.,the remaining vapors are completely condensed to yield an aqueousformaldehyde solution containing from about to by weight offormaldehyde. This latter condensate may then be converted toparaformaldehyde by cooling. The condensatesinV thepar- Vtial condenserspreferably have aresidence'time tioirisA also undesirableinthat "it ls'quite 'soft 55 in said apparatus of less than about one second.

solution of formaldehyde. maining in separators Ii and I3 is returned toThe condensate obtained from the first partial condenser during theinitial partial condensation and comprising a dilute aqueous solution offormaldehyde containing to 20% by weight of formaldehyde is pumped backto the pressure column in which the initial distillation is effectedtogether with the incoming raw formaldehyde feed. The condensates fromthe second and third partial condensations are combined and arepreferably returned to the system so as to constitute the reflux to thepressure column. The pressure tower in which the initial distillation is`effected discharges, as residue, Water containing less than 1% byweight of formaldehyde which is then discarded. Higher boilingimpurities, such as hydroxy acetone or glycols are also discharged fromthe pressure tower. Not only does my novel process yield concentratedformaldehyde of a high degree of purity but by effecting the partialcondensation in a plurality of steps f in series, a higher yield ofpurified formaldehyde is obtained than when the partial condensation .iscarried out in a single step.

In order further to illustrate my invention the following example isgiven:

Example Referring now to Fig. 1 of the drawing, the dilute raw aqueousformaldehyde solution to be concentrated is introduced through a feedline 3 into a pressure column 4 in the form of a 12.2% by weight aqueoussolution. The raw aqueous formaldehyde solution is entered at a rate of99.0 parts by weight per hour. rihe pressure column 4 in which theinitial distillation takes place operates under a pressure of to 40pounds per square inch gage and a temperature of 130 to 140 C. Thevapors from pressure column 4 pass through a pressure reducer 5 Wherelthe pressure is dropped to about 10 pounds per square inch and thevapors then passed through a partial condenser 6. 32.2% by weight of thevapors is condensed in partial condenser 6 and, after separation of theVapor from the liquid condensate in a cyclone separator 1, thecondensate, comprising 30.1 parts by Weight per hour vof a 8.1% byweight aqueous solution of formal- ,dehyde, is returned to pressurecolumn 4 by means of a pump S and a line 9 which joins line 3 throughwhich the raw formaldehyde feed is introduced. The uncondensed vapor, inan amount of 63.6 parts by Weight per hour of a 30.7% by weight aqueoussolution of formaldehyde, is then passed through a second partialcondenser I0 where a further partial condensation of 28.1% of theinitial vapors is effected.

The condensate is removed from the mixture of condensate and vapor in acyclone separator II,

.the uncondensed vapor being passed to a third partial condenser I2 inwhich the final partial condensation takes place. The uncondensed vaporremaining is separated from the condensate by means of a cycloneseparator I3. The vapor ,leaving separator I3 comprises 14.9 parts byWeight per hour of an 80.5% by Weight aqueous pressure column 4 throughlines I4 and I5 which unite in a line I6 and feed the condensate,comprising 48.7 parts by weight per hour of a 15.6%

.by weight aqueous formaldehyde solution, to a pump I'I. The latterforces the condensate back to pressure column 4 through a line I8. Therecycled condensate acts as reux in the pressure column.

The condensate re- The concentrated formaldehyde vapor, comprising about14.9 parts by weight per hour leaves separator I3, passes through a lineI9 into a total condenser 20 of novel design. Condenser 20 is cooled bymeans of water or other cooling means circulating through a jacket 2|.Condenser 20 comprises a plurality of superimposed trays Which aregenerally indicated by reference numeral 22. Trays 22 each comprise abubble cap 23 provided with a downcomer 24, the design being such thatboth the liquid and vapor flow are in theV same downward direction asshown by the arrows in Fig. 2. Total condensation is effected incondenser 20 and an 80.5% by Weight solution of formaldehyde isobtained. The vapor flow through the system of partial condensers,cyclone separators and the total condenser 20 is maintained by means ofa steam jet 26, the pressure maintained in said system being at about 10to about 15 pounds per square inch absolute. The rapid passage of thecondensing yapor and condensate through my novel condenser limits theresidence time of the concentrated formaldehyde solution in thecondenser so that a residence time of about 2 to not more `than about 10minutes and a condensate temperature of about to 100 C. is achieved andprecipitation of paraformaldehyde is thereby effectively avoided. A pump21 passes the condensate from condenser 20 to a soaking vessel 25wherein the concentrated aqueous formaldehyde is allowed to remain at atemperature of to 1.30" C. for about 10 to 60 minutes prior to beingcooled and solidified. The concentrated aqueous formaldehyde from vessel25 is then passed to a drum cooler 28 where the solution is cooled onthe surface of the drum and the paraforrnaldehyde which precipitatesscraped therefrom in the form of flakes.

My novel process is a continuous one and enables paraformaldehyde of asubstantially uniform degree of polymerization and formaldehyde contentto be readily obtained. v It is to be understood that the foregoingdetailed description is given merely by way of illustration and thatmany variations may be made therein without departing from the spiritofxmy invention.

Having described my invention, what I desire 'to secure by LettersPatent is:

1. Process for the production of paraformaldehyde, which comprisessubjecting a dilute aqueous solution of formaldehyde to distillation un-.'-der a pressure of about 30 to 45 pounds per square inch gage,reducing the pressure on the vapor distillate to a pressure of about 5to 15 pounds per square inch absolute, passing the vapors through aplurality of cooling zones in each of which a partial condensation ofthe vapor takes place, removing the condensed liquid from each zonerapidly so that the residence time of the condensed liquid in thecooling zone is less than `about one second, the total amount of liquidso ycondensed being at least about 70% of the vapors from the initialpressure distillation, returning the condensate from the initialcondensation to the pressure distillation together with the incomingdilute aqueous formaldehyde feed, returning vthe combinedfcondensatefrom the remaining partial condensation to the pressure distillation asreflux, totally condensing the vapors .remaining after the nal partialcondensation, and cooling the condensate.

y 2. AProcess forthe production of paraformalderhyde, whichv comprisessubjecting a dilute aqueous solution of formaldehyde to distillationunder a pressure of about 30 to 45 pounds per square inch gage, reducingthe pressure on the vapor distillate to a pressure of about 5 to 15pounds per square inch absolute, concentrating the vapors by subjectingthe same to a plurality of partial condensations, whereby about 30 to50% by weight of the vapors are condensed during the initial partialcondensation, about 20 to 40% by weight condensed during a secondpartial condensation and about to 30% by weight condensed during thefinal partial condensation, returning the condensate from the initialcondensation to the pressure distillation together with the incomingdilute aqueous formaldehyde feed, returning the combined condensate fromthe remaining partial condensations to the pressure distillation asreflux, totally condensing the vapors remaining after the final partialcondensation, and cooling the condensate.

3. Process for the production of paraformaldehyde, which comprisessubjecting a dilute aqueous solution of formaldehyde to distillationunder a pressure of about 30 to 45 pounds per square inch gage, reducingthe pressure on the vapor distillate to a pressure of about 5 to 15pounds per square inch absolute, concentrating the vapors by subjectingthe same to a plurality of partial condensations, whereby about 30 to50% by weight of the vapors are condensed during the initial partialcondensation, about 20 to 40% by weight condensed during a secondpartial condensation and about 10to 30% by weight condensed during thefinal partial condensation, returning the condensate from the second andfinal condensations to the pressure distillation as reflux, totallycondensing the vapors remaining after the nal partial condensation,allowing the condensate to soak at about 100 to 120 C. for 10 to 60minutes and then cooling the condensate to yield paraformaldehyde.

4. Process for the concentration of formaldehyde, which comprisessubjecting a dilute aqueous solution of formaldehyde to distillationunder a pressure of about 30 to 45 pounds per square inch gage, reducingthe pressure on the vapor distillate to a pressure of about 5 to 15pounds per Square inch absolute, passing the vapors through a pluralityof cooling zones in each of which a partial condensation of the vaportakes place, removing the condensed liquid from each zone rapidly sothat the residence time of the condensed liquid in the cooling zone isless than about one second, the total amount of liquid so condensedbeing at least about 70% of the vapors titi ous solution of formaldehydeto distillation una der a pressure of about 30 to 45 pounds per squareinch gage, reducing the pressure on the vapor distillate to a pressureof about 5 to 15 pounds per square inch, concentrating the vapors bysubjecting the same to a plurality of partial condensations, wherebyabout 30 to 50% by weight of the vapors are condensed during the initialpartial condensation, about 20 to 40% by Weight condensed during asecond partial condensation and about 10 to 30% by Weight condensedduring the nal partial condensation, returning the condensate from theinitial condensation to the pressure distillation together with theincoming dilute aqueous formaldehyde feed, returning the combinedcondensate from the remaining partial condensations to the pressuredistillation as reflux, and totally condensing the vapors remaining'after the final partial condensation.

6. Process for the concentration of formaldehyde, which comprisessubjecting a dilute aqueous solution of formaldehyde to distillationunder a pressure of about 30 to 45 pounds per square inch gage, reducingthe pressure on the vapor distillate to a pressure of about 5 to l0pounds per square inch absolute, concentrating the vapors by subjectingthe same to a plurality of partial condensations, whereby about 30 to50% by Weight of the vapors are condensed during the initial partialcondensation, about 20 to 40% by weight condensed during a secondpartial condensation and about 10 to 30% by weight condensed during thefinal partial condensation, returning the condensate from the second andfinal condensations to the pressure distillation as reflux, and totallycondensing the vapors remaining after the nal partial condensation.

References Cited in the le of lthis patent UNITED STATES PATENTS NumberName Date 1,871,019 Walker Aug. 9, 1932 1,905,033 Bond Apr. 28, 19332,257,780 Bludworth Oct. 7, 1941 2,411,436 Kopp Nov. 19, 1946 2,425,669Brock Aug. 12, 1947 2,432,405 Gerhold Dec. 9, 1947 2,498,206 Greenwaldet al. Feb. 21, 1950 2,527,655 Pyle et al. Oct. 31, 1950 2,529,622Michael Nov. 14, 1950 FOREIGN PATENTS Number Country Date 503,180Germany July 24, 1930 479,255 Great Britain Feb. 2, 1938 OTHERREFERENCES Industrial and Engineering Chemistry, July 1940, vol. 32, No.7, pages 1016-1018.

Industrial and Engineering Chemistry, April 1948, vol. 40, No. 4, pages661-672.

Formaldehyde, Walker, (c) 15944 by Reinhold Publishing Corp., New York,New York, pages 58 to 63.

1. PROCESS FOR THE PRODUCTION OF PARAFORMALDEHYDE, WHICH COMPRISESSUBJECTING A DILUTE AQUEOUS SOLUTION OF FORMALDEHYDE TO DISTILLATIONUNDER A PRESSURE OF ABOUT 30 TO 45 POUNDS PER SQUARE INCH GAGE, REDUCINGTHE PRESSURE ON THE VAPOR DISTILLATE TO A PRESSURE OF ABOUT 5 TO 15POUNDS PER SQUARE INCH ABSOLUTE, PASSING THE VAPORS THROUGH A PLURALITYOF COOLING ZONES IN EACH OF WHICH A PARTIAL CONDENSATION OF THE VAPORTAKES PLACE, REMOVING THE CONDENSED LIQUID FROM EACH ZONE RAPIDLY SOTHAT THE RESIDENCE TIME OF THE CONDENSED LIQUID IN THE COOLING ZONE ISLESS THAN ABOUT ONE SECOND, THE TOTAL AMOUNT OF LIQUID SO